It has been customary in the electronic article surveillance industry to apply EAS markers to articles of merchandise. Detection equipment is positioned at store exits to detect attempts to remove active markers from the store premises, and to generate an alarm in such cases. When a customer presents an article for payment at a checkout counter, a checkout clerk either removes the marker from the article, or deactivates the marker by using a deactivation device provided to deactivate the marker.
Known deactivation devices include one or more coils that are energizable to generate a magnetic field of sufficient amplitude to render the marker inactive. One well known type of marker (disclosed in U.S. Pat. No. 4,510,489) is known as a "magnetomechanical" marker. Magnetomechanical markers include an active element and a bias element. When the bias element is magnetized in a certain manner, the resulting bias magnetic field applied to the active element causes the active element to be mechanically resonant at a predetermined frequency upon exposure to an interrogation signal which alternates at the predetermined frequency. The detection equipment used with this type of marker generates the interrogation signal and then detects the resonance of the marker induced by the interrogation signal. According to one known technique for deactivating magnetomechanical markers, the bias element is degaussed by exposing the bias element to an alternating magnetic field that has an initial magnitude that is greater than the coercivity of the bias element, and then decays to zero. After the bias element is degaussed, the marker's resonant frequency is substantially shifted from the predetermined interrogation signal frequency, and the marker's response to the interrogation signal is at too low an amplitude for detection by the detecting apparatus.
In one conventional deactivation device, a drive circuit occasionally applies a drive signal having a decaying AC waveform to a coil or coils. The drive circuit is triggered to generate the drive signal in response to a button or switch actuated by the checkout clerk, or by circuitry which detects the presence of an active marker.
More recently, in co-pending patent applications that are commonly assigned with the present application, it has been proposed to eliminate the triggering mechanism and to drive the deactivation device coil or coils with a continuous wave AC sinusoid having constant amplitude (or a periodically interrupted version of such a signal), as disclosed in application Ser. No. 08/794,012, filed Feb. 3, 1997; or with discrete single cycles of an AC sinusoid, also with constant peak amplitudes, as disclosed in application Ser. No. 09/110,508, filed Jul. 6, 1998. In the case of both of these coil excitation schemes, the required decay in the signal actually applied to the EAS marker is accomplished by sweeping the marker past the deactivation coils so that the field applied to the marker is attenuated as the marker exits the region in which the field is radiated.
The disclosures of the '012 and '508 patent applications are incorporated herein by reference.
Although sweeping markers past the deactivation device coils can work quite effectively, it is sometimes desirable to provide a deactivation device which does not require the marker to be swept. An example of such a deactivation device is the so-called "bulk" deactivator disclosed in U.S. Pat. No. 5,781,111. (The '111 patent has a common inventor and a common assignee with the present application, and the disclosure thereof is incorporated herein by reference.)
In addition, enhanced energy efficiency is another desirable goal for marker deactivation devices.